Incorrect example in drush onlyone-list command definition

Problem/Motivation

The example in the onlyone-list command related to the not available option is now like this:

'drush onlyone-list not-available' => dt("Lists the content types which cannot be configured in 'Only One node' mode."),

But the correct argument is: notavailable

Proposed resolution

Change the not-available argument to notavailable

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.